1. Field of the Invention
The present invention relates generally to electrophotographic printing devices, and, more particularly, to the operation of the heating units in a fuser roll and control of standby temperatures of the fuser roll between print jobs.
2. Description of the Related Art
In the electrophotographic (EP) imaging process used in printers, copiers and the like, a photosensitive member, such as a photoconductive drum or belt, is uniformly charged over an outer surface. An electrostatic latent image is formed by selectively exposing the uniformly charged surface of the photosensitive member. Toner particles are applied to the electrostatic latent image, and thereafter the toner image is transferred to the media intended to receive the final permanent image. The toner image is fixed to the media by the application of heat and pressure in a fuser.
A fuser is known to include a heated roll and a backup roll defining a nip through which the media passes. During the fusing process, it is necessary that sufficient heat be applied to the toner particles so that the toner is permanently affixed to the media. If the temperature is too low, the under-fused toner can smear, transfer to printing apparatus surfaces or otherwise degrade the quality of the currently printed media and even subsequent printings. Adequate fusing temperatures are quite high, and therefore it is necessary to maintain the heated roll at an elevated standby temperature between print jobs, so that printing is not delayed excessively while the fuser is heated to an acceptable fusing temperature. It is also necessary that the heated roll is not heated excessively, since excessive fuser roll temperatures can cause hot offset and wrap of the media on the fuser roll. The need to maintain the fuser temperature above the minimum and below a maximum temperature, which may be a relatively small temperature window, can cause delays as the fuser roll is warmed or allowed to cool as necessary.
It is known to provide fuser rolls made of aluminum cores with elastomeric coverings. While such structures work well for a number of reasons, including sheet release properties of the elastomeric cover, the relatively poor thermal conductivity through the structures provides other challenges in maintaining desired printing temperatures without unduly delaying print jobs.
When relatively small print jobs are processed, such as jobs entailing one to five printed pieces of media, a condition of overshoot can occur, in which the fuser roll becomes hotter than desired after the print job is completed. As media passes through the fuser nip, heat is transferred from the surface of the heated roll to the media. The temperature control system in the printing apparatus detects the decreasing temperature, and activates heating lamps or other devices to provide additional heat to the fuser roll. Heat is absorbed by the aluminum core of the roller and is transferred from the core to the elastomeric layer, increasing the surface temperature of the roller. However, because of the relatively poor thermal conductivity of the elastomeric layer, and the large heat capacity of the aluminum core, heat transfer can be slow. When a small print job occurs, the job may complete and media cease passing through the fuser at the end of the print job before significant heat transfer occurs from the core to the elastomer layer. Since passing media is no longer sinking heat from the elastomer, the excess energy stored in the core transfers to the elastomer, producing a substantial rise in surface temperature after a small print job has been completed. The rise or overshoot may be well in excess of the maximum temperature for successful operation.
When overshoot occurs, the roller must be allowed to cool before another print job starts, to avoid the possible hot offset and media wrap problems mentioned previously. This can cause a significant time lag between print jobs, as the roller is allowed to cool. The only cooling mechanism available to the fuser is free convection cooling, and since the elastomeric roll cover has relatively poor thermal conductivity, cooling is slow and may require six to ten seconds for each one degree Centigrade of cooling required. Therefore, it is desirable that overshoot be eliminated or reduced to minimize print quality and first copy time delays.
Relatively complex control algorithms have been developed for processing print jobs, and to control heating to minimize overshoot at the end of the print job. Such algorithms have worked well for large print jobs; however, for shorter print jobs, the predictive nature of such algorithms does not have sufficient time or information to adequately affect the magnitude of overshoot at the end of the small print job. Such algorithms require some forehand knowledge of job size and the number of pages remaining to be printed. However, many current print managers for the print engine and raster image processor (RIP) interface may only cue up four sheets at a time. If more sheets are in the pipeline, the engine is not aware of them until a page is emptied from the cue. Also, the RIP sends information as it receives it, not when it has completely processed the job. Thus, the engine must continuously check to see if the RIP has sent new information or pages. The uncertainty of the number of pages left in the page manager pipeline or the need to hold off declaring the end of a print job results in less than optimal performance when the overshoot reduction algorithm functions for short print jobs.
Since the standard print job in many applications of printing apparatuses is three sheets or less, a need exists to minimize fuser temperature overshoot and to improve the first page print performance of printers processing print jobs of five sheets or less.
What is needed in the art is a control sequence or algorithm to minimize temperature overshoot and to control fuser roll standby temperatures so as to improve first page performance of electrostatic printing apparatuses printing small print jobs.